In systemd v209, released over two years ago, the various libsystemd-*
libraries (libsystemd-journal.so, libsystemd-login.so, libsystem-daemon.so,
libsystemd-id128.so) were merged into a single libsystemd.so library to
reduce code duplication and avoid cyclic dependencies.

This refactoring is driven by the need to be able to do true queued RPC calls
(where each successive call uses the result of the previous).

Currently, such queueing of requests is done by assigning them special "magic"
tag numbers, which are then intercepted in one big switch() statement and acted
upon. This (aside from making code greatly unclear) effectively makes each such
queue a singleton, because state passing is restricted to global variables.

We refactor RpcClient to assign an unique tag to each remote call, and then
abstract all the call<->response matching with Qt's future/promise mechanism.

Finally, we introduce a "RPC request queue" class (RpcQueue) which is built on
top of QFutureWatcher and C++11's <functional> library. This class maintains
a queue of functions, where each function receives an RPC response, does
necessary processing, performs another call and finally returns its future.